This invention relates to a proximity detector, and especially to a ferrous-gear-tooth Hall-transducer, or other magnetic-field-to-voltage transducer, capable of detecting the leading and trailing gear tooth edges of an adjacent rotating ferrous gear, or other magnetic articles. The term "magnetic article" as used herein applies to magnetized bodies, ferrous bodies and other bodies having a low magnetic reluctance that tend to alter the ambient magnetic field.
In the patent U.S. Pat. No. 5,442,283, issued Aug. 15, 1995 there is described a Hall-voltage slope-activated proximity-detector capable of detecting the rising and falling edges of an adjacent rotating gear tooth. This proximity-detector type detector includes an integrated circuit Hall detector mounted to a pole of a magnet, and includes a circuit for tracking a slope of a Hall voltage (e.g. corresponding to the approach of a passing gear tooth) and briefly holding the ensuing peak voltage before producing an output signal indicating the onset of the following Hall-voltage slope of opposite direction (e.g. corresponding to the approach of a valley between two gear teeth). The Hall voltage holding circuit includes a capacitor and circuit means for controllably leaking charge out of or into the capacitor for preventing false tripping of a comparator that provides the pulse output signal. The holding voltage of the capacitor thus has a droop which leads to increasing loss of holding accuracy as the speed of gear tooth passage becomes slower, and therefore the detector has a minimum gear teeth speed at which accurate detection is possible.
It is an object of the invention to provide a magnetic article proximity detector that is capable of accurate operation over a wide range of speeds of magnetic-article passage down to zero.